SiH4 is an excellently suitable starting material which, following further purification if necessary, can be pyrolysed in order to separate very pure, semiconductor-quality silicon. The demand for high-purity silicon is growing strongly and so does the demand for pure silane whose excellent suitability for the production of high-purity silicon is increasingly recognized and utilized.
Among the processes for producing silane described in the literature, the production from trichlorosilane by means of disproportionation is advantageous from an economic point of view. It is known that amines, in particular tertiary amines and their hydrochlorides and quaternary ammonium chlorides, both in liquid form (DE 3 500 318 A1) and in solid form, e.g. bound to solid carriers, can be used as catalysts in order that the disproportionation of trichlorosilane be accelerated in an economically advantageous manner. Amines bound to solid carriers (U.S. Pat. Nos. 4,701,430, 5,026,533, DE 3 500 318 A1, DE 3 311 650 C2, DE-OS-2 507 864) are preferably used since in this way impure amines can be prevented from being dragged into the reacting gaseous/liquid silane/chlorosilane phase.
Liquid catalysts, as selected in some of the processes described, are disadvantageous in that they are gradually dragged out of the reactor since they can in no case be separated completely from the reaction products. The amounts of catalyst dragged along cause problems in subsequent process steps, or also in upstream process steps in case of a circulation system, since they can collect at certain places of the system and e.g. catalyse undesired reactions there. In addition, a liquid catalyst cannot be successfully distributed in the column as evenly as possible, but it will concentrate locally due to its specific steam pressure. This problem is not at all solved, but at the most reduced, by using two catalysts having different boiling points as suggested in DE 3 500 318 A1.
As a rule, the disproportionation of trichlorosilane is carried out in several steps, for example in two steps. Attempts have been made for individual steps of disproportionation to take place according to the principles of reactive distillation. Reactive distillation is characterized by combining reaction and separation by means of distillation in one apparatus, particularly in a column. By continually removing the lowest-boiling component in each element of space by means of distillation, an optimum gradient between the state of equilibrium and the actual content of lower-boiling components or the lowest-boiling component, respectively, is maintained at any time so that a maximum reaction velocity results. For example, JP 01 317 114 indicates a reactive distillation process for the step of disproportionating dichlorosilane to silane and a trichlorosilane/silicon tetrachloride mixture. DE OS-2 162 537 also indicates a reactive distillation process for the aforesaid disproportionation step. In addition, DE OS-2 162 537 also shows a reactive distillation process for the step of disproportionating trichlorosilane to dichlorosilane and silicon tetrachloride.
DE OS-2 507 864 discloses a process for producing silane which is characterized in that trichlorosilane is introduced into a bed of an anion exchange resin which is not soluble in the reaction medium and contains tertiary amino groups or quaternary ammonium groups on a carbon atom and the temperature of the resin bed is maintained such that trichlorosilane is disproportionated into products which ascend in the bed, on the one hand, and silicon tetrachloride which condenses and flows to the bottom of the column, on the other, and in that the temperature at the upper part of the bed is maintained above the boiling point of silane and below the boiling point of monochlorosilane and silane which is virtually free from chlorosilanes is obtained from the bed.
The aforesaid process distinguishes itself from the other known processes in that                (1) it is a single-stage process as regards equipment, i.e. the desired enriched products silane and silicon tetrachloride can be removed at different places of one and the same apparatus so that relatively few equipment and a reduced amount of energy are required, in that        (2) it permits the products silane (in concentrations between 96 and 98% SiH4) and silicon tetrachloride (in concentrations e.g. between 70 and 80% SiCl4) to be produced in relatively high concentrations with no further auxiliary aggregates being needed, in that        (3) only minimum amounts of impurities are dragged from the catalyst into the reaction mixture thanks to the solid insoluble catalyst (hereinafter referred to as catalytically active solid matter) and a considerably lower separation effort for separating off the catalysts is required, compared to liquid soluble catalysts, and the collection of volatile, liquid catalysts in certain sections of the column is strictly avoided, and in that        (4) the amount of energy required for separating the silanes or chlorosilanes forming during the individual stages of equilibrium of the disproportionation process is reduced due to the principle of reactive rectification.        
However, a serious disadvantage of the aforesaid process described in DE OS-2 507 864 consists in that the amount of energy used for separating the silanes or chlorosilanes has to be carried away completely at a very low temperature level corresponding to the condensation temperatures. According to DE OS-2 507 864, the temperature at the top of the column has to be set below the condensation temperature of monochlorosilane SiH3Cl, while the temperature in the trichlorosilane SiHCl3 inlet area has to be set such that it enables trichlorosilane to be evaporated. Thus the energy required for evaporating the various chlorosilanes and silane in the individual sections of the column is finally carried away at a temperature below the condensation temperature of monochlorosilane, i.e. below −50° C. down to −120° C., depending on pressure. As is generally known, carrying away heat at low temperature levels is costly and causes additional energy consumption, with costs and energy required increasing the lower the temperature of the cooling medium has to be set.